By incorporating a turbocharger, comprising a compressor driven by a turbine, into an engine of a vehicle, the efficiency and power output of the engine may be improved. The forced induction of extra air into a combustion chamber of the engine proportionally induces the combustion of additional fuel, producing more power than obtained from air intake at ambient pressure. The pressurized, or boosted, air is typically heated during compression and if combusted directly, may increase the risk of engine knock. Thus, boosted air is typically cooled by flowing the air through a charge air cooler (CAC) before entering the engine intake manifold, a process that also increases the density of the air and improves intake throttle response.
The air compression provided by the compressor of the turbocharger is enabled by the rotation of the turbine. Typically, at least a portion of the exhaust gas is routed to the turbine and the expansion of the exhaust gas causes the turbine to spin. Since the turbine is mechanically coupled to the compressor, as the turbine spins up, so does the compressor. However, there may be a delay between the time when an increase in torque is demanded and when the corresponding boost pressure is provided by the compressor. The delay, also referred to as turbo lag, is due to the turbine's inertia and friction when operating at low engine loads, and corresponds to a duration required for the engine to generate sufficient exhaust gas (e.g., exhaust flow and temperature) to spool up the turbine to drive the compressor. During this turbo lag, boosted engine performance may be degraded.
Various approaches have been developed to address the issue of turbo lag including providing an alternative pathway for supplying air to the engine. One example approach is shown by Sealy et al. in U.S. Pat. No. 6,561,169. Therein, a charge air management system is disclosed wherein a first air duct supplies air at ambient pressure to an intake manifold and a second air duct flows cool, boosted air, from downstream of a charge air cooler (CAC), to the intake manifold. Air flow through the first air duct or the second air duct is controlled via a valve based on the engine's speed and load. During low loads, air is delivered via the first duct. During higher loads, cooled, dense air is delivered via the second duct and the CAC.
However, the inventors herein have recognized potential issues with such systems. There may be conditions where the air provided via the second duct does not improve boosted engine performance. In the approach of Sealy, closing of the valve during a low load condition allows cool and dense compressed air to be held within the second duct, and released when the valve is opened during a subsequent high load condition. Since the charge air management system of Sealy is adapted for infrequent demands on the turbocharger, the CAC is not operated during low load conditions to improve fuel economy. Consequently, based on ambient conditions, as well as a duration over which the compressed air is held in the second duct without operation of the CAC, the boosting potential of the compressed air may degrade. For example, during conditions when the ambient temperature or humidity is high, the density of the compressed air held in the second duct may decrease, reducing the engine's throttle response when the compressed air is subsequently released. If the CAC is operated while air is retained in the sealed second duct, the fuel economy benefit of the boosted engine may be lost.
In one example, the issues described above may be addressed by a method for reducing turbo lag comprising; at higher engine load, drawing cool compressed air into an engine via an air intake passage and at lower engine load, drawing ambient air into the engine via a duct while retaining cooled compressed air in the air intake passage. Additionally, compressed air is released from the air intake passage based on heat transferred to the compressed air during the lower engine load. In this way, boost pressure response time following a transient increase in torque demand can be improved.
As one example, a dual pathway air induction system may be adapted to an engine of a vehicle. The induction system may include a first air passage coupled to an intake manifold via a first throttle, the first air passage configured to deliver fresh air at ambient pressure to engine cylinders at low engine speed and loads. The induction system may further include a second air passage coupled to the intake manifold via a second throttle, the second passage including a turbocharger compressor and a CAC for delivering cool, compressed air to the engine cylinders during high engine speeds and loads. The first passage may be coupled to the second passage via a coupling, such as a T-body. Additionally, flow to each passage may be controlled via a splitter valve (e.g., a proportional valve) or via respective first and second throttle valves. Responsive to an operator tip-in, flow through the second passage may be increased while flow through the first passage is decreased. Responsive to a subsequent operator tip-out, CAC operation is disabled, flow through the first passage may be increased, and flow through the second passage is closed to trap an amount of cool, compressed air within the second passage. A rate of temperature rise of the trapped air is estimated while the second passage is closed. For example, heat transfer to the trapped air may be estimated as a function of ambient temperature, humidity, volume of air trapped, boost level attained before the second throttle was closed, etc. If the amount of heat transfer exceeds a threshold, such as when the inferred temperature of the trapped air exceeds a threshold temperature, the engine controller may open the second throttle and release the trapped warm air, even if a higher torque was not demanded. The first throttle may be correspondingly closed and one or more engine operating parameters may be adjusted (e.g., spark timing may be retarded from MBT) to reduce torque transients.
In this way, by trapping cool, compressed air within a duct, the duct can be used as a boost pressure reservoir that rapidly provides boost pressure to engine cylinders during a tip-in. As such, this reduces turbo lag. The technical effect of discharging the trapped compressed air while torque demand is low responsive to a rise in the inferred temperature of the trapped air is that boost performance of the engine can be maintained elevated. Specifically, only compressed air that is cool and dense, and therefore capable of improving throttle response when discharged, is trapped inside the duct. By discharging trapped air that is warm and less dense during conditions when boost demand is low, the adverse effect of the warm air on throttle response is reduced. The duct can be replenished with fresh air that is rapidly cooled during subsequent CAC operation. In this way, boosted engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.